1. Field of the Invention
This invention relates generally to a moisture curable hot melt composition and, more specifically, to a reactive wax-containing moisture curable hot melt composition that sets quickly.
2. Description of the Related Art
Hot melt compositions (e.g., adhesives, coatings and sealants) are a solid at room temperature, melt to a viscous liquid when heated to a moderate temperature, and are applied molten to an appropriate substrate. The molten composition then cools and solidifies to form a bond to the substrate. One important parameter in characterizing the performance of hot melt compositions is set time.
"Set time," as used herein and as described more fully below, refers to the time required for the hot melt composition to no longer permit repositionability once the composition has been applied. In a typical adhesive bonding operation, the adhesive is applied to the substrate and a second member is placed on the substrate to be bonded thereto by the hot melt composition. For a certain period of time after the hot melt composition has been applied, the second member may be moved, adjusted or repositioned. Once that period of time (i.e., the set time) elapses, the second member can no longer be repositioned.
Another important parameter in assessing the performance of hot melt compositions is the initial strength or "green strength" of a bond made therewith. Green strength, as explained more fully hereinbelow, is often measured by the ability of the bond to support a given weight for at least a certain time. Therefore, green strength and set time are frequently related since set time can be regarded as the time that must pass before the bond can support a given weight for a certain time.
In certain instances it is particularly desirable to employ a hot melt composition having a short set time; that is, a hot melt composition that rapidly achieves a certain green strength. One example is an automated assembly line. For instance, on an automated production line for motor vehicle lamp assemblies, lenses for the vehicle side lights and tail lights are typically adhesively bonded to lamp housings. The adhesive possesses a certain set time that permits the lens to be manually repositioned if it was not accurately mounted when first applied.
However, once the lens is properly positioned, the adhesive should set quickly and achieve a high green strength. Until the set time is exceeded and the green strength reached, the lens may be inadvertently jarred free and fall off the lamp housing as the lamp assembly progresses along the production line, unless the components are clamped together.
Current production methods allow for accurate placement of the lens on the lamp housing. Consequently, it is desirable to have a quick setting hot melt composition so that the speed of the assembly line and hence the number of completed units can be increased.
At least two distinct and readily differentiatable classes of hot melt compositions are recognized: conventional hot melt compositions and reactive or curing hot melt compositions.
Conventional hot melt compositions typically comprise an ethylene/vinyl acetate copolymer, a tackifying resin, and a wax. Waxes are usually included to reduce the viscosity of the molten material. While conventional hot melt compositions are easily used, readily handled, and have excellent initial strength, they tend to form bonds having a low ultimate strength. Consequently, conventional hot melt compositions are most often used in applications requiring only low strength bonds such as bookbinding operations and the sealing of cardboard cases and cartons.
"A-C.RTM. Polyethylene Functional Waxes for Hot Melt Adhesives" (presented at the June, 1988 TAPPI Hot Melt Symposium) reports the use of functional waxes, such as oxidized polyethylene waxes, in ethylene/vinyl acetate and rubber block/copolymer hot melt adhesives. The functional waxes were incorporated into the hot melt adhesives as either partial or complete replacements for nonfunctional microcrystalline and paraffin waxes or as a partial replacement for the base copolymer. Reportedly, the functional waxes substantially increased adhesion to a wide variety of substrates while still dramatically reducing viscosity. Other benefits which are said to be derived from the use of functional waxes in these hot melt adhesives include faster set speeds.
European Patent Application No. 0,451,352 "Process to Produce Cationically Active Waxes," published Oct. 16, 1991 discloses, as an intermediate product, a wax (based on Fischer-Tropsch or polyethylene oxidates) carrying isocyanate groups. Reportedly the wax can be used in melt adhesives.
Reactive or curing hot melt compositions typically comprise an isocyanate-terminated polyurethane prepolymer (sometimes referred to as a polyurethane prepolymer or simply as a prepolymer). Once applied in a molten state to a substrate, these hot melt compositions cool and solidify to provide an initial bond strength and eventually react with ambient moisture (or moisture obtained from the substrate) in a curing reaction that results in a thermoset, high ultimate strength bond. While reactive hot melt compositions are known for their excellent final strength (i.e., after the moisture curing reaction), they are often regarded as suffering from slow set times and low initial strength.
U.S. Pat. No. 3,931,077 "Reactive Hot Melt Adhesive Composition And Method For Use Thereof," issued Jan. 6, 1976 to T. Uchigaki et al. discloses a reactive hot melt adhesive composition comprising 20 to 73 wt. % of a urethane prepolymer having terminal isocyanate groups, 2 to 25 wt. % of a thermoplastic resin (e.g., an ethylene/vinyl acetate copolymer, an ethylene/acrylic acid copolymer, an ethylene/acrylate copolymer, atactic polypropylene or a polyethylene terephthalate linear polymer), and 25 to 55 wt. % of a tackifier. The relative amounts of the different ingredients are apparently selected so as to provide a good balance of initial and ultimate strength properties. Reportedly, the hot melt compositions have a low viscosity at a relatively low temperature without requiring the addition of viscosity improving agents such as wax, which have been used in conventional hot melt adhesive compositions to decrease viscosity but with a concomitant decrease in adhesive strength.
U.S. Pat. No. 5,115,073 "Rapidly Crystallizing Polyurethane Systems," issued May 19, 1992 to W. Meckel et al. discloses a polyurethane system based on polyisocyanates and at least two polyesters having different melting points. These systems apparently rely on the presence of crystalline polyesters to enable the hot melt compositions to build up a certain strength immediately after application of the hot melt. Reportedly, the crystallization rate can be greatly accelerated by adding small quantities of a high melting polyester polyol (namely 2 to 30 parts by weight of a polyester diol having a melting point of 65.degree. C. to 150.degree. C. and a molecular weight of 600 to 8000) without the properties of these systems as a whole being adversely affected. Some of these polyester polyols can add significant cost to the manufacture of the hot melt composition.
U.S. Pat. No. 5,173,538, "Moisture-Curing, Polyurethane Hot-Melt Compositions," issued Dec. 22, 1992 to H. G. Gilch et al. This patent criticizes reactive polyurethane hot melt materials that set by crystallization for having a low initial peel strength immediately after bonding and a slow setting rate that depends on the crystallization rate of the polyesters. A quick setting, moisture curing, polyurethane hot melt adhesive composition based on polyether prepolymers is disclosed. One polyether based prepolymer has a glass transition point above room temperature and a second prepolymer (which may be based on a polyether, a polyester, or another hydroxyl-terminated polyol) has a glass transition point below room temperature. These compositions are believed to be based on the use of amorphous prepolymers rather than crystalline prepolymers.
International Patent Publication No. WO 91/15530 "Hot Melt Moisture-Cured Polyurethane Adhesive," published Oct. 17, 1991 includes a segmented thermoplastic polyether-polyester elastomer having both hard and soft segments. Reportedly, the use of this material enables the resulting moisture curable hot melt to achieve excellent green strength. While waxes are mentioned as an optional component, no distinction is made between different types of wax.
Consequently, there remains a need for an economical hot melt composition that sets quickly without adversely affecting other important properties such as the green strength or the ultimate strength of the bond. Such compositions would find considerable utility in a variety of applications, for example, the automated assembly procedure described above. In this instance, the lamp assembly production line could be operated more quickly because the hot melt composition would rapidly set to a high green strength. The risk that the lens would come loose, even at accelerated production rates, would be reduced.